Highly active crystalline aluminosilicate catalysts of intermediate pore size have become of considerable commercial importance. One such crystalline aluminosilicate is ZSM-5, described and claimed in U.S. Pat. No. 3,702,886 to Argauer et al. This catalyst has been found useful for a number of hydrocarbon and other conversion reactions. For example, U.S. Pat. No. Re. 28,398 to Chen et al. describes its application in catalytic dewaxing; U.S. Pat. No. 4,097,367 to Haag et al. describes its use in converting olefinic hydrocarbons; U.S. Pat. No. 3,856,871 to Haag et al. describes its use for for xylene isomerization; and U.S. Pat. No. 3,928,483 to Chang et al. describes its use for the conversion of dimethyl ether to gasoline. U.S. Pat. No. 3,911,041 to Kaeding et al., U.S. Pat. Nos. 4,025,571 and '572 to Lago, U.S. Pat. No. 4,025,575 to Chang et al. and U.S. Pat. No. 4,052,479 to Chang et al. describe its use in converting methanol and/or dimethyl ether to olefins, to aromatic hydrocarbons, or to mixtures thereof. These are but a few of the large number of patents that have issued which are concerned with the applications of this versatile catalyst. The foregoing patents are incorporated herein by reference as background material.
It is known that the ZSM-5 type catalysts, in general, are more resistant to deactivation induced by the accumulation of coke than the large pore zeolites such as Zeolite X, Y, and mordenite. It is also known that ZSM-5 type catalysts deactivated by the accumulation of coke may be regenerated by burning off the coke at high temperature in an oxygen-containing gas. It is also known that ZSM-5 type catalysts are relatively resistant to permanent damage from exposure to high temperature in the presence of steam, which conditions prevail when a coked catalyst is regenerated by burning off the coke. In spite of these advantageous properties, deactivation due to coke deposition is noted after sufficient time on stream, and regeneration, with exposure to high temperature, is practiced. Such regeneration, if not carefully conducted, or if repeated a sufficient number of times, can lead to catalyst damage, i.e. a loss of catalytic activity which is not recovered by the usual regeneration. Exposure of ZSM-5 type catalyst to high temperature in the presence of product steam, such as is encountered in the conversion of methanol to gasoline, also leads to catalyst damage after a sufficient period of time. Operations involved in catalyst manufacture such as burning off the organic template, or conversion of the ammonium to the hydrogen form by calcination, if not carefully controlled, also may lead to catalyst damage. Furthermore, a number of applications, such as that described in U.S. Pat. No. 4,283,584 to Chester et al., require that the catalyst be steamed to reduce its catalytic activity to a prescribed value. Accidental oversteaming of the catalyst, i.e. steaming for too long a time or at too high a temperature, also produces a catalyst which may be regarded as damaged.
There is evidently a need for a simple and reliable method for reclaiming ZSM-5 type catalysts that have become damaged either in the course of manufacture or during use.
U.S. Pat. No. 3,493,490 to Plank et al. describes a method for reactivating damaged cracking catalysts made with large pore zeolites. In this method, the damaged catalyst is treated with an anionic reagent selected from the group consisting of liquid water at a temperature above 212.degree. F. or a solution containing a hydroxyl ion from a non-alkalimetal compound. U.S. Pat. No. 3,533,959 to Miale et al. describes a method for reclaiming damaged large pore zeolite cracking catalysts by treatment with a chelating agent at a pH between about 7 and 9. U.S. Pat. No. 4,468,475 to Kuehl discloses a method for hydrothermally activating a high-silica crystalline zeolite composited with alumina binder. Zeolites with silica-to-alumina ratios greater than about 500:1 or, still better, over 2000:1, may be activated. None of these patents disclose or suggest the present invention.
U.S. Pat. No. 4,324,696 to Miale describes a method for preparing a superactive HZSM-5 catalyst by treatment with steam at elevated temperature followed by base exchange of the resultant steamed product with an ammonium salt. However, the method described is not applicable to a damaged catalyst, and there appears to be no suggestion to treat the catalyst with a liquid medium. These same comments apply to U.S. Pat. No. 4,374,296 to Haag et al.
The term "deactivated" is used herein in a generic sense to refer to loss of activity for any reason, such as the accumulation of coke or catalyst poisons, or possibly alteration of the crystal in some fashion. In general, deactivation due to coke and poisons is reversible by regeneration with air or hydrogen. Deactivation that is not reversed by regeneration, although not fully understood, is often ascribed to some alteration of the zeolite crystal. The term "damaged" as used herein is specifically reserved for deactivation not reversed by the usual regeneration procedures.